linux-kernel - Re: [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS

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Message-ID: <CALCETrVZXBRtQidZJ7=CXD_AkKkjL0JkJQAeZz44rk5ckpLDfw@mail.gmail.com>
Date:	Thu, 30 Apr 2015 12:17:35 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	linux@...elenboom.it
Subject: Re: [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen
 PV guests

On Thu, Apr 30, 2015 at 12:08 PM, Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
> Commit 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor
> attribute issue") makes AMD processors set SS to __KERNEL_DS in
> __switch_to() to deal with cases when SS is NULL.
>
> This breaks Xen PV guests who do not want to load SS with__KERNEL_DS.
>
> Since the problem that the commit is trying to address would have to be
> fixed in the hypervisor (if it in fact exists under Xen) there is no
> reason to set X86_BUG_SYSRET_SS_ATTRS flag for PV VPCUs here.
>

Seems reasonable.

Have you run the test case on a Xen PV guest on AMD?  It's possible
that Xen is affected, since the old accidental workaround that we
deleted was in the vdso and probably would have worked on Xen.

--Andy
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